The present invention relates to vias metallization of printed circuit boards and more particularly, the present invention relates to enhancing the throwing power in the electroplating of the vias.
In view of the continuous advancements in semiconductor performance together with rapid expansion of the demand for sophisticated electronic devices, particularly in mobile and portable applications, the need for fabricating circuit feature of a small size and interconnection substrates is substantially increasing. Multi-layered printed circuit boards are now using high aspect ratio through hole vias and blind vias openings for high density interconnections. Uniform plating distribution inside these vias represents a main issue for PCB reliability.
New ways to improve mass transport and new electrolyte additives have increased the uniformity of electrodeposition inside blind vias.
Blind vias having a diameter (d) of 150 microns or less and an aspect ratio (AR) (see equation [1]) greater than 1 are difficult to plate properly using conventional techniques. Currently, in order to enhance copper deposition inside blind vias, the technique of reverse pulse plating or the use of complex chemical solutions have been proposed and used. These processes are not without their limitations, despite the fact that they are useful. As is known, industrial plating solutions can be extremely complex and can contain up to four organic additives. Additive concentrations require constant monitoring and are usually adjusted because many of these additives are destroyed or sacrificed during the plating process. Another limitation is that the solution, subsequent to use is environmentally unfriendly and can result in expensive disposal costs.
Regarding a pulse step position, this process also employs complex chemical solutions and involves a significant capital investment since the method does not employ the same current rectifiers typically associated with conventional DC plating. One of the other limitations to this process is that health problems could be an issue for the operators since reverse pulse systems emit strong magnetic fields.
As is known in fluid dynamics, ultrasonic agitation enhances mass transfer and this technique can be applied to electrochemistry. This was proposed by Walker in, Chemistry in Britain, 1990, pp. 251-254.
Although there have been advances in the electroplating of the circuit board vias, these methods remain complex to control and run. There is a need in the industry to have a method which is easier to operate and which provides for a similar or more efficient electrodeposition. The present invention satisfies these needs.
One object of the present invention is to provide an improved system and method for enhancing the throwing power in an electroplating cell.
The method is particularly well adapted for industrial applications of PCB plating for high production levels with uniform application of the plating material.
According to a further object of one embodiment, there is provided a method for electroplating blind vias or through holes in a printed circuit comprising the steps of: providing a printed circuit board having blind vias or through holes therein; providing a plating cell containing solution for plating in the vias of the printed circuit board, the plating cell further including anodes; providing ultrasonic vibration means for vibrating the plating solution during electrodeposition; and vibrating the solution to electroplate the blind vias or through holes.
It has been found that ultrasonic agitation in accordance with the present invention substantially increased the microthrowing power improvement for small interconnection blind vias.
The ultrasonic treatment may occur using transducers operating in the range of 20 kHz to 60 kHz suitably positioned within the plating bath. For purposes of the instant application, copper electrodeposition was employed and to this end the transducers were positioned within titanium hollow containers in view of the fact that the containers are chemically inert, under certain conditions, to the plating bath and do not interfere with the electroplating procedure. It will be appreciated by those skilled in the art that the container may comprise any suitable material and this will depend on the environment in which the transducers are employed and the nature of the compounds in the solution.
It is envisioned that the ultrasonic transducers are positioned directly within the cell at a suitable location to induce hydrodynamic cavitation within the cell and thus increase the uniformity of deposition within the blind vias. To augment the electrodeposition efficiency, chemical additives may be used in combination with the ultrasonic agitation. Suitable additives are known to those skilled in the art.
Other known methods may be combined with the ultrasonic treatment of the solution such as agitation of the PCB board or substrate to be treated in addition to the ultrasonic treatment of the solution. Further, it is clearly envisioned that other forms of treatment including reverse pulse deposition could also be used in combination with the ultrasound treatment.
Another object of one embodiment of the invention is to provide a method of plating blind vias in integrated circuits, comprising the steps of: providing a printed circuit board to be plated; providing a plating cell containing solution for plating in the blind vias or through holes of the printed circuit board, the cell further including anodes; providing ultrasonic vibration means for vibrating the plating solution during electrodeposition; introducing a gas adjacent the printed circuit board for localized agitation of the plating solution around the printed circuit board; and vibrating the solution to electroplate the blind vias or through holes.
According to a further object of one embodiment of the present invention, there is provided a system for electroplating vias in a printed circuit board, the system comprising: an electroplating cell having a pair of anodes; means for supplying power to the cell; an electrochemical solution; a substrate for receiving material to be electroplated; and ultrasonic vibration means for vibrating the solution.
Having thus described the invention, reference will now be made to the accompanying drawings illustrating preferred embodiments.